Method for detecting a gas using an infrared gas analyser and gas analyser suitable for carrying out said method

ABSTRACT

The invention relates to a method for detecting a test gas that may be present at a measuring location, using an infrared gas analyser ( 1 ) that comprises a cuvette ( 2 ), an infrared light source ( 3 ), an infrared detector ( 4 ), and two gas lines ( 15, 17 ). Said gas lines supply gases to the infrared gas analyser and one of said lines is adapted to take up a measuring gas at a measuring location that may contain a test gas. The second line ( 17 ) is adapted to take up gas from the surroundings of the measuring location (reference gas), which gas may contain a test gas background that is to be taken into consideration when detecting the test gas taken up at the measuring location. In order to improve the sensitivity of the analyser it comprises only one cuvette ( 2 ) and the measuring gas taken up at the measuring location and the reference gas are fed to the cuvette of the infrared gas analyser in such a manner that they are alternately present in the cuvette.

[0001] The present invention relates to a method for detecting a testgas that may be present at a measuring location, said test againsthaving the characteristics of patent claims 1 and 2, as well as aninfrared gas analyser suited for performing these methods.

[0002] Processes and devices of this kind are known from DE-A-199 11260. These are in particular suited for applications in the area ofsniffer leak detection. In sniffer leak detection, an object under testcontaining a working gas is scanned by means of a sniffer tip into whichthe measurement gases are drawn. If a leak is present, the working gaswill escape to the outside. This is then supplied via the sniffer tip tothe gas detector. If the working gas is not active within the infraredrange, then a test gas which is active in the infrared range is added tothe working gas in the object under test. In this instance, themeasurement gas penetrating a possibly present leak consists of amixture of working gas and test gas. If the working gas is alreadyitself active in the infrared range (a halogen gas, for example), thenitself may act as the test gas (or measurement gas).

[0003] In the area of sniffer leak detection there exists the problemthat the sniffer tip will not only suck in test gases escaping from apossibly present leak (measurement location), but also gases from thevicinity of the measurement location. If these already contain lowconcentrations of the test gas, for example from previously determinedleaks or from filling stations of a production line, these will also berecorded by the gas detector. At high test gas backgrounds this cancause erroneous measurements, i.e. that leak-tight objects under testare “detected” as being faulty.

[0004] In order to avoid disadvantages of this kind, it is proposed inDE-A-199 11 260 that the test gas concentration of the gas taken up atthe measurement location be compared with the test gas. concentration ofthe reference gas (gas taken up in the vicinity of the measurementlocation) with the aid of two cuveftes, a measurement cuvefte and areference cuvefte, so as to take into account interfering influences.The use of two separate cuvettes with one or two precisely modulatedinfrared light sources is not only involved engineering-wise, but hasalso some disadvantages. One of these disadvantages is that the cuvettesdo not change their properties in a uniform manner. They can collectcontaminants in a non-uniform manner; when employing two infrared lightsources these may age differently. In the document mentioned as beingstate-of-the-art it is proposed that it is also possible to employ onlyone infrared light source. However, this necessitates splitting of thebeam. Such beam splitting and also merging of the beams detailed also insaid document (for the purpose of employing only one infrared detector)results in relatively high losses (50% approx.) through which inparticular the sensitivity of the gas analyser is impaired.

[0005] It is the task of the present invention to simplify methods anddevices of the kind affected here, in particular with respect toimproving their sensitivity.

[0006] This task is solved through the present invention by thecharacteristics of the patent claims.

[0007] In the methods and devices according to the present inventiononly one cuvette is required. Contamination and changes to the entirebeam path (lamp—cuvette—detector) have an equal effect during both themeasurement gas cycle and during the reference gas cycle. The singleinfrared light source must not necessarily be modulated, the modulationis attained by exchanging the gas. Above all, the esspecially precisemodulation required for the state-of-the-art can be omitted, i.e. slowerand brighter light sources may be employed, this being especiallybeneficial to the sensitivity of the analyser. Also beam splitting andbeam merging are omitted. The single cuvette is at times filled with themeasurement gas possibly containing the test gas, and at times with thereference gas (or additionally with the reference gas). If theconcentration of the test gas in the measurement gas, i.e. at themeasurement location, is greater than the concentration of the test gasin the reference gas, the infrared detector will detect an alternatingsignal which is a measure for the difference in concentration. In this,it does not matter whether or not a test gas background is actuallypresent. Finally, from this there results the advantage that in theinstance of the present invention the zero line (measurementgas=reference gas) is considerably more stable compared to thestate-of-the-art, because in the instance where measurement gas andreference gas are identical during the gas exchange, no modulationcomponent is generated. In contrast to this, in the instance of thestate-of-the-art, two relatively large modulated signals needed tocompared with each other which generally involves interfering componentsof significant magnitude.

[0008] Within the scope of the present invention it is beneficial toprovide means for monitoring the operation of the infrared gas analyseraccording to the present invention, so as to avoid erroneousmeasurements caused by contamination or faults. Contaminants may notonly impair the gas supply; also the sensitivity of the gas analyseritself decreases with increasing contamination.

[0009] For the purpose of avoiding an impaired gas supply, it isproposed to utilise the pressure in the gas supply lines as themeasurement quantity for monitoring the flow. Monitoring the operationof the gas analyser itself is performed in accordance with the presentinvention such, that the infrared light source is modulated with areference frequency, the signal of which is continuously monitored atthe infrared detector (preferably with its own lock-in processing).

[0010] Finally, it is particularly expedient to employ a gas lamp as theinfrared light source said gas lamp containing—at least a share—of thetest gas. Compared to incandescent lamps, gas lamps offer the generalbenefit of being brighter (improved utilisation of the luminous power)and that they can be switched—modulated—faster. Since an infrareddetector has a limited signal to noise ratio, the resolution of theanalyser will increase with the brightness of the light source.

[0011] As gas lamps, flashlights, gas discharge lamps, glow lamps oralike may be employed. Since these are operated with the gas which is tobe detected, filters can be omitted. Moreover, a wide range of theabsorption spectrum may be utilised since the generated spectrum and thespectrum of the measurement gas substantially agree. The share of theutilised luminous power is high, through which there results asignificantly improved gas selectivity of the infrared detector. Theconversion to a different type of gas can be performed simply in that agas lamp with a different type of gas is employed.

[0012] Further advantages and details of the present invention shall beexplained with reference to the embodiments depicted in drawing FIGS. 1to 6.

[0013] Depicted in drawing FIGS. 1 to 4 are infrared gas analysers withdifferent facilities for supplying the measurement gas and the referencegas to a single cuvette. This is performed in the embodiments

[0014] according to drawing FIG. 1 with the aid of a valve system,

[0015] according to drawing FIG. 2 via an intermediate volume with alinearly moving piston,

[0016] according to drawing FIG. 3 via an intermediate volume with arotary/oscillating piston and

[0017] according to drawing FIG. 4 via a throttle (measurement gas) anda valve (reference gas).

[0018] Drawing FIG. 5 depicts an embodiment for monitoring the flowthrough a sniffer.

[0019] In drawing FIG. 6 an embodiment is presented which is equippedwith means for checking its operation.

[0020] In all drawing figures the infrared gas analyser is generallydesignated as 1, its cuvette as 2, the infrared light source located onone face side as 3, the detector located on the opposite face side as 4,the connected electronic subassembly (amplifier/filter) as 5, a theretoconnected further electronic subassembly (lock-in amplifier) serving thepurpose of signal processing as 6, and a display as 7. Lock-inprocessing is commonly performed in software within a microcontroller;only for the purpose of being able to provide a better explanation, aseparate block 6 is depicted. The cuvette 2 is equipped with connections8 and 9 at its respective face sides. Via said connections themeasurement gas and the reference gas is supplied, respectivelydischarged, in accordance with the methods described below.

[0021] The face sides of the cuvette 2 are represented by dashed linesin each instance. These shall indicate that the substantially gas-tightside walls of the cuvette are capable of passing infrared light in thearea of the face sides. As infrared sources of light and infrareddetector, facilities may be employed as detailed in DE-A-199 11 260.

[0022] In all examples of embodiments, sniffer leak detection has beenselected as the application subject to the present invention. 11designates an object under test which is to be analysed for the presenceof leaks, said object having a leak 12. In this instance, the locationof the leak is the measurement location. The sniffer tip 13 serves thepurpose of taking up the measurement gas which, owing to the presence ofleak 12, contains test gas. Via a line 15 connected to the sniffer, thetaken in measurement gas flows to cuvefte 2. Opening 16 in the hose line17 serves the purpose of taking up gas from the vicinity of the sniffertip (reference gas). This gas may contain a test gas background whichduring the determination of the concentration of the gas flowing out ofthe leak 12 shall be taken into account.

[0023] In the embodiment according to drawing FIG. 1 a control valve 21serves the purpose of alternately supplying the measurement gas and thereference gas. It is so designed that either line 15 or the line 17 isconnected to the inlet connection 8 of the cuvette 2. The gas in eachinstance flows through the cuvette in the axial direction and exits itthrough the discharge connection 9 which is linked to a supply or vacuumpump 22. This pump will define, depending on its pumping speed, thevelocity of the flow of the gases to be analysed within the cuvette 2.

[0024] The gas exchange is preferably performed periodically. To thisend, the control unit 23 of the control valve 21 is linked via the line24 to the lock-in amplifier 6. The reliance on the basically knownlock-in technology has the advantage that the wanted signal can befiltered in a frequency- and a phase-selective manner. Thus interferingsignals are suppressed very effectively. In the instance where also theinfrared light source 3 shall be modulated synchronously with the gasexchange, this light source is also linked to the lock-in amplifier 6.This variant is indicated by the dashed line 24′¹⁾. Lock-in processingincluding control may also be performed by a microcontroller system withsuitable software.

[0025] In the embodiment according to drawing FIG. 2 there is locatedbetween the cuvette 2 and the lines 15, 17 a preferably cylindricallydesigned intermediate volume 25—as depicted—in which there is located apiston 27 joined to a crank drive 26. The lines 15 and 17 each open outin the area of the opposing face sides of the intermediate volume 25. Inthese areas the intermediate volume is also linked to the inletconnections 8 and 10 at the cuvette 2. The piston 27 forms in theintermediate volume two separate chambers 28, 29. Chamber 28 serves thepurpose of accepting and discharging the measurement gas, chamber 29serves the purpose of accepting and discharging the reference gas. Theto-and-fro motion of the piston 27 ²⁾ effects an alternating supply ofmeasurement gas and reference gas into the cuvette 2. The supplied gasesexit the cuvette via the discharge connection 30 arranged expedientlyapproximately at the middle of the cuvette 2, said connection beingconnected to the pump 22 (drawing FIG. 1). In comparison to theembodiment in accordance with drawing FIG. 1, the gas is exchanged incuvette 2 more rapidly and more completely.

[0026] Also in the embodiment in accordance with drawing FIG. 3 there ispresent an intermediate volume 25 with the chambers 28, 29. These areformed by a housing 31 having a circular cross-section, arotary/oscillating piston 32 having the cross-section of a semicircle,and a radial separating wall 33. The chambers 28, 29 are linked via thelines 35, 36 to the connections 8, 10 ³⁾ of the cuvette 2. Lines 15, 17open out into lines 35, 36 in such a manner, that an oscillating motionof the piston 32 produced by drive 37, alternately fills the cuvette 2with measurement gas and with reference gas. These are gases exit thecuvette 2 via the middle connection 30.

[0027] The rate at which the periodic gas exchange in the cuvette 2 iseffected in the instance of the embodiments in accordance with drawingFIGS. 1, 2 and 3 is defined by the lock-in amplifier 6. This amplifieris linked in each instance via the line 24 to the valve 21, the crankdrive 26 or the drive at 37 of the oscillating piston 32. Rates of onesecond to ⅙ second per period have been found to be expedient.

[0028] In the embodiments in accordance with drawing figure is 1 to 3the cuvette 2 is filled during a first period with the measurement gasand during second period with the reference gas. In contrast to this inthe instance of the embodiment in accordance with drawing FIG. 4, themeasurement gas is supplied continuously into the cuvette 2 via the line15. The reference gas is only periodically supplied specifically via thevalve 41 with its control unit 43, said valve being incorporated in line17. The opening and closing times of this valve are also defined by theclock rate defined by the lock-in amplifier 6.

[0029] During measurement operations, the reference gas and themeasurement gas, respectively reference gas only, also alternately flowthrough the cuvette 2. If the measurement gas contains test gas escapingfrom a leak, the detector 4 supplies the desired alternating signal.

[0030] Since leak rate sensitivity depends on the flow of themeasurement gas respectively the flow of the test gas (high sensitivityat low measurement gas flow), it is expedient to equip line 15 with athrottle 42, so rated that the flow of the measurement gas will fill thecuvette with the measurement gas within half a period. In contrast tothis, the flow of the reference gas may be high since practically enoughreference gas is available.

[0031] Moreover, there exists in all embodiments the possibility ofselecting a shorter measurement cycle for the reference gas compared tothe measurement cycle for the measurement gas. Thus dead time isreduced, faster and/or more sensitive measurements are possible.

[0032] As already mentioned, the infrared light source needs not to bemodulated with the clock of the lock-in amplifier since already the gasexchange effects the desired modulation. However, there exists thepossibility of modulating the infrared light source 3 in additionsynchronously to the gas exchange so as to attain sharper rising edgesin the measurement signals. Without modulation, the edges of themeasurement signal and depend on how rapidly the gas is exchanged.

[0033] Drawing FIG. 5 depicts, based on the embodiment in accordancewith drawing FIG. 1, how the flow through sniffer at high sensitivitycan be checked/monitored during operation. This is performed with theaid of a differential pressure sensor 45 which is linked at the level ofthe valve 21 to the lines 15, 17. The measured differential pressuresignal is supplied via an amplifier 46 to processing logic 47. Whenanalysing this differential pressure signal in a phase selective mannerwith respect to the switching frequency, one obtains information whetherthe flow in one or the other sniffer line or in both sniffer lines haschanged. Also blockages in the line of the pump, respectively a fault inthe pump are detected.

[0034] A different possibility of monitoring the differential pressurecan be implemented with the aid of flow sensors. However, this is moreinvolved compared to the method detailed above.

[0035] Drawing FIG. 6 also depicts, based on the embodiment inaccordance with drawing FIG. 1, how the operation of the gas analyser 1operating at high sensitivity can be checked/monitored. This may beapplied in a solution in which the signal at the sensor is modulatedwith a fundamental frequency fg (supplied by the lock-in amplifier 6)and where the frequency-selective receiving unit determines the leakrate.

[0036] In the solution in accordance with drawing FIG. 6 there isprovided a second lock-in amplifier 51 supplying a reference clocksignal at a frequency fr. The inputs of the lock-in amplifiers 6 and 51are connected to each other via line 52. At the output of the lock-inamplifier 51 there is connected a processing unit 53 which passes on itsinformation to the display 7. Via the line 54 with the amplifier 55 thelock-in amplifier 51 is linked to the infrared light source 3.

[0037] In the embodiments depicted in drawing FIG. 6, the gas analyser 1is monitored such that the infrared lamp 3 is modulated with an(additional) signal of a different frequency. In order to avoidinterferences with the wanted signal fg, the selection of a frequencyof, for example, fr=2.5 fg is expedient. The magnitude of the modulationmust be selected such that this signal can be analysed well in afrequency-selective manner in the receiving unit. One then obtains ameasurement signal with the frequency fg and a reference signal with thefrequency fr which may be evaluated independently of each other. If,owing to a fault, system sensitivity should be reduced, this is detectedby the decreasing amplitude of the signal component at fr (also when noleak is measured). Based on this it is possible to define the limits foran error message.

[0038] Processing is performed in block 53. Such processing can alsoserve the purpose of adapting the calibration factor at decreasingsensitivity, thereby increasing measurement accuracy.,

[0039] In the drawing figures and descriptions separate blocks aredepicted in each instance said blocks being components of the circuitsemployed. Expedient is the usage of integrated systems. For example, forlock-in processing, control and processing of the measurement andcontrol signals, a microcomputer or a microprocessor circuit withattendant software may be employed.

1. Method for detecting a test gas that may be present at a measuringlocation, using an infrared gas analyser (1) that comprises a cuvette(2) accepting the gases to be analysed, an infrared light source (3)located at one face side of the cuvette, an infrared detector (4)located at the other face side of the cuvette, the signals from theinfrared detector serving the purpose of determining test gas recordedat the measurement location, as well as two gas lines (15,17) servingthe purpose of supplying gases to the infrared gas analyser and one ofsaid lines is adapted to take up a measuring gas at a measuring locationthat may contain test gas and where the second line (17) is adapted totake up gas from the surroundings of the measuring location (referencegas), which gas may contain a test gas background that is to be takeninto consideration when detecting the test gas taken up at the measuringlocation, wherein only one cuvette (2) is present, that the measuringgas taken up at the measuring location and the reference gas are fed tothe cuvette of the infrared gas analyser in such a manner that they arealternately present, and where in the instance that at the measurementlocation test gas is taken up the concentration of which in themeasurement gas is higher than the test gas concentration in thereference gas, where the alternating signals supplied by the infrareddetector serve the purpose of determining the increased test gasconcentration.
 2. Method according to claim 1, wherein a control valve(21) is employed which alternately connects the line (15) and the line(17) to a connection (8) at the cuvette.
 3. Method according to claim 1,wherein an intermediate volume (25) with a piston (27,32) locatedbetween the cuvette (2) and the lines (15,17) is employed for thealternately supplying reference gas and measurement gas to cuvette (2).4. Method for detecting a test gas that may be present at a measuringlocation, using an infrared gas analyser (1) that comprises a cuvette(2) accepting the gases to be analysed, an infrared light source (3)located at one face side of the cuvette (2), an infrared detector (4)located at the other face side of the cuvette, the signals from theinfrared detector serving the purpose of determining test gas. recordedat the measurement location, as well as two gas lines (15,17) servingthe purpose of supplying gases to the infrared gas analyser and one (15)of said lines is adapted to take up a measuring gas at a measuringlocation that may contain test gas and where the second line (17) isadapted to take up gas from the surroundings of the measuring location(reference gas), which gas may contain a test gas background that is tobe taken into consideration when detecting the test gas taken up at themeasuring location, wh rein only one cuvette (2) is present, that themeasuring gas taken up at the measuring location and the reference gasare fed to the cuvette (2) of the infrared gas analyser (1) in such amanner that the measurement gas taken up at the measurement locationconstantly flows through the cuvette (2) whereby reference gas is addedat times and where in the instance that at the measurement location testgas is taken up the concentration of which is higher than the test gasconcentration in the reference gas, where the alternating signalssupplied by the infrared detector serve the purpose of determining theincreased test gas concentration.
 5. Method according to one of theclaims 1 to 4, wherein for signal processing the lock-in technology isemployed.
 6. Method according to one of the claims 1 to 5, wherein themeasurement cycle is arranged to be longer than the reference cycle. 7.Method according to claim 1 to 6, wherein the operation of the gasanalyser is constantly monitored.
 8. Method according to claim 7,wherein in addition to a gas exchange at a fundamental frequency fg, theinfrared light source (3) is (at times) modulated with a referencefrequency fr.
 9. Method according to one of the claims 1 to 8, whereinthe gas flow is constantly monitored.
 10. Method according to claim 9,wh rein the pressure in the supply lines (15,17) is measured and wherethe difference between these pressures serves as the measurementquantity.
 11. Method according to one of the above claims, wherein alsothe infrared light source (3) is modulated synchronously to the gasexchange.
 12. Device for performing a method with the characteristics ofclaim 1, wherein it is equipped with means (21 to 23; 25 to 29; 31 to37) which admit measurement gas taken up at the measurement location andthe reference gas into the cuvette of an infrared gas analyser in such amanner that they are alternately present in the cuvette.
 13. Deviceaccording to claim 12, wherein the means consist of a control valve (21)which alternately connects the line (15) containing the measurement gasand the line (17) containing the reference gas to the cuvette. 14.Device according to claim 12, wherein the means comprise an intermediatevolume (25) with two separate chambers (28,29) through which with theaid of a piston (27,32), measurement gas and reference gas isalternately supplied to the cuvette (2).
 15. Device according to claim12, 13 or 14, wh rein the cuvette is equipped with a dischargeconnection (9,30) to which a supply/vacuum pump (22) is connected. 16.Device for performing a method with the characteristics of claim 4, wherin a control valve is present in the line (17) serving the purpose ofsupplying the reference gas.
 17. Device according to claim 16, wherein athrottle (42) is present in the line (15) serving the purpose ofsupplying the measurement gas.
 18. Device for performing a method withthe characteristics of claim 5, wherein a lock-in amplifier (6) is acomponent of the electronics, the line (24) carrying the clock signal isconnected to the valves (21), to the drive (26,37) of the pistons(27,32), to the valve (41), and possibly connected to the infrared lightsource (3).
 19. Device for performing a method with the characteristicsof claims 7 and 8, wherein a further lock-in amplifier (51) is provided,the line 54 carrying the clock signal for the reference frequency beingconnected to the infrared light source.
 20. Device according to claim19, wherein the reference frequency fr is greater, preferably greater bya factor of approximately 2.5 compared to the fundamental frequency fg.21. Device according to claim 18 or 19, wherein the second lock-inamplifier is linked to a processing unit (53) serving the purpose ofdetecting a system fault.
 22. Device according to claim 21, wherein theprocessing unit is also employed for adapting a calibration factor. 23.Device for performing a method with the characteristics of the claim 9or 10, wherein a differential pressure sensor (45) is connected to thelines (15,17) and where the output of the pressure sensor is connectedto processing logic (47).
 24. Device according. to claim 23, wherein theprocessing logic contains its own lock-in amplifier.
 25. Deviceaccording to one of the above claims, wherein for the purpose of lock-inprocessing, control and processing of the measurement and controlsignals, a microcomputer or a microcomputer circuit with attendantsoftware is employed.